Port stays clear trumps buoy room on beat.

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RULE 

Part 2 - When Boats Meet

Section C - At Marks and Obstructions

Rule 18.1 When Rule 18 Applies

...it does not apply

(b) between boat on opposite tacks when the proper course at the mark for one but not both is to tack.

Buoy room rules do not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them... 

PURPOSE

Defend the lead of a starboard boat at a port rounding.